Improvement of Surface Morphology and Superconducting Properties of YBCO Coated Conductor on Ni-W Tapes

High-quality YBCO coated conductors have been deposited on NiW tapes by pulsed laser deposition process. Although thin YBCO films (less than 0.5
$\mu\hbox{m}$
) have a very high critical current density
$J_{\rm c}$
of
$4.0 \times 10^{6}\ \hbox{A/cm}^{2}$
(at 77 K, in zero magnetic field), when YBCO film thickness increased over 1.0
$\mu\hbox{m}$
, the
$J_{\rm c}$
values of YBCO decreased quickly. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations showed that thick YBCO films had micrometer-scale grains and relatively broad grain boundaries that blocked superconducting current paths. In order to control the grain size and grain boundary width of YBCO films as well as improving superconducting properties under applied magnetic field, substrate modification and optimization of cap layer were applied. AFM and SEM images showed that YBCO films on the optimized substrate have smooth surface and higher superconducting critical current density. The maximum
$I_{\rm c}$
of YBCO film increased from 280 A/cm to 480 A/cm and
$J_{\rm c}$
of 1-
$\mu\hbox{m}$
-thick YBCO film increased from
$2.3 \times 10^{6}\ \hbox{A/cm}^{2}$
to
$3 \times 10^{6}\ \hbox{A/cm}^{2}$
.